1. Field of the Invention
The present invention relates to a technique of using plasma to etch an organic film used for a semiconductor device.
2. Description of the Related Art
When a cylinder hole or contact hole is formed in a semiconductor device manufacturing process, a mask having a multilayer structure is used because of the large aspect ratio and large depth of such a hole. An example of such a mask is one which is formed with the steps of: stacking an organic film and an SiO2 film (silicon oxide film) on an insulating film subjected to etching, for example, an SiO2 film; and performing dry development on this laminated film using a photoresist.
An example of the dry development is explained below with reference to FIGS. 12(a) to 12(c). Reference numeral 1 in FIG. 12A denotes a laminated structure which includes an organic film 12 composed mainly of carbon, an SiO2 film 13, and a photoresist (PR) film 14 stacked in this order from the bottom upward. Photolithography is performed on the photoresist film 14 to form a resist pattern 15. First, a gas for etching such as CF4 is converted to plasma. The SiO2 film 13 is then etched along the resist pattern 15 with the plasma to form a mask pattern 16 on the SiO2 film 13 (FIG. 12(b)). Next, O2 gas alone or a mixed O2 gas with H2 gas or N2 gas added thereto is converted to plasma. Then, the organic film 12 is etched (ashed) through the mask pattern 16 to form a mask pattern 17 for etching an SiO2 film 11 which is the underlayer of the organic film 15 (FIG. 12(c)).
The organic film 15 is decomposed into CO2, H20, and the like by oxygen radicals. The oxygen radicals have a strong propensity to etch the organic film 12 in an isotropic manner, that is, in the lateral direction as well as in the depth direction. As shown in FIG. 12(c), the mask pattern 17 may form into an arch shape in which the vertical cross-section of the pattern laterally spreads, called a bowing shape, which reduces the degree of verticality of the side walls 18 of the mask pattern 17. As a result, when the SiO2 film 11, a film to be etched, is etched with the SiO2 film 13 and the organic film 12 as a mask, the anisotropic shape of the organic film 12 is transferred onto that film to be etched, resulting in a degraded shape of a hole formed in the film etched (SiO2 film 11).
In order to prevent the mask pattern 17 from having the bowing shape, in some cases, a CF- or CH-based gas as well as an O2 gas are converted to plasma, and the organic film 12 is etched. At the same time, chemical compounds produced from these gases are adhered to the side walls 18 of the mask pattern 17 to form protection films for protecting the organic film 12 against the oxygen radicals. Still in those cases, occasionally, the lateral etching caused by the oxygen radicals cannot be sufficiently restrained, resulting in the bowing shape.
In the case where the organic film 12 itself is etched as a film to be etched, the photoresist film 14 is, in some cases, directly formed on the organic film 12 without forming the SiO2 film 13 thereon, and the organic film 12 is etched along the resist pattern 15. However, since the photoresist film 14 is composed of organic substances, the photoresist film 14 is also etched when the organic film 12 is etched, resulting in a distorted pattern shape. This prevents the inherent shape of the resist pattern 15 from being normally transferred onto the organic film 12. For example, the shape of the hole in the organic film 12, which is a film subjected to etching, may become bow-shaped or bottom-spread. With the above-mentioned laminated structure 1, the SiO2 film 13, which is an inorganic film, provided between the photoresist film 14 and the organic film 12 can prevent the upper side of the mask pattern 17 from being exposed to the oxygen plasma. However, if the thickness of an inorganic film provided is small, the upper side of the mask pattern 17 is likely to be exposed to the oxygen plasma as in a case where there is no inorganic film. In this case, the bowing shape is likely to occur. Therefore, in order to control the shape of the mask pattern 17, it is necessary to consider a suitable thickness also for the inorganic film.
Japanese Patent Laid-open, JP-A-2002-9058, discloses a technique for forming protection films on the side walls of a pattern when an organic film is etched. However, the protection films do not contain silicon, and thus the technique cannot solve the above-mentioned problem.